Molten Salt Electrolytic Production Of Carbon-based Materials And Their Electrochemical Performances

Carbon materials have taken an important role in energy storage and other fields due to their outstanding physicochemical properties.Controllable synthesis of high-performance carbon-based materials is one of the key research hotspots in these fields.Herein,molten salt electrochemical synthesis of new-type carbide-derived carbon(CDC)materials and their applications in supercapacitor have been investigated.Based on the electrochemical etching method and new integrated molten salt electrochemical synthesis-etching strategy,a variety of high-performance carbon materials including nodular-like mesoporous carbon,porous carbon nanospheres,core-shell N-doped carbon,and two-dimensional(2D)carbon nanosheets have been synthesized through the preliminary design of carbide precursor and fine controllable electrochemical parameters.The electrosynthesis mechanism,physicochemical properties and electrochemical performances of these carbon materials have also been investigated in detail.The main conclusions are shown as follows:(1)Firstly,special micro/nano-structured carbide materials such as Si C,Cr₇C₃ and Cr₂Al C have been facilely synthesized via molten salt electro-deoxidation process with low-cost metal oxides/carbon mixtures as raw materials.The elements migration route and reactions mechanism from Si O₂/C to Si C are investigated by systematically analyzing cathodic and anodic products.The results show that the oxygen in Si O₂/C precursor can be ionized and migrates to graphite anode.The generation process of Si C nanoparticles mainly involves chemical/electrochemical combination reactions between Si O₂ and Ca O in melts,electrochemical reduction reaction,in-situ carbon combination reaction and grain growth process.The electrolytic preparation of Cr₇C₃and Cr₂Al C has also been investigated.It is found that the cathodic reactions mainly include electrochemical combination/deoxidation process,in-situ carbonization reaction and intermediate combination/growth process.The oxygen content of the cathode gradually decreases with the increase of electrolysis time,and the oxygen content in Cr₇C₃ after being electrolyzed for 24 h is about 3500 ppm.Furthermore,it is confirmed that the preferentially generated Cr_xC_y functions as the template and promotes the growth of nodular-like Cr₂Al C.The research work in this part provides the theoretical basis and experimental exploration for the controllable synthesis of various micro/nano-structured metal carbide materials via molten salt electrolysis method.(2)The feasibility of molten salt electrochemical preparing carbide-derived carbon and the physicochemical properties of products have been further investigated with micro/nano-structured metal carbides as precursors.The experimental study combined with theoretical calculation prove that the metal/metal-like atoms with high electrochemical activity can be effectively etched and removed from the carbide lattice under the applied electric field force,therefore,Si C-CDC nanosphere with typical amorphous structure can be synthesized.The obtained Si C-CDC possesses a specific surface area of 585 m²/g and abundant nanopores with pore volume of 0.42 cm³/g.Furthermore,Si C-CDC as electrode material for supercapacitor delivers a high specific capacity of 120 F/g at 0.5 A/g,with the capacitive retention rate of 98.3%,showing excellent capacitive performance.Meanwhile,the molten salt electro-deoxidation process has been facilely integrated with electro-etching process.Cr₇C₃ and Cr₂Al C with special micro/nano-structure have been firstly electro-synthesized from Cr₂O₃/C and Cr₂O₃/Al₂O₃/C precursor via molten salt electrolysis method.Then,dual micro/nano-structured Cr₇C₃-CDC and Cr₂Al C-CDC are achieved by directly in-situ etching the synthesized carbides.The electrochemical etching process and the electrolytic products have been carefully investigated,revealing that the production of carbide derived carbon is a conformal transformation process.Different carbides possess intrinsic influence on the microstructure of derived carbon materials,which provides an experimental guidance for the subsequent adjustment of high-performance carbide derived carbon materials.(3)Based on the above investigation of molten salt electrolytic production process,molten salt electro-etching is further combined with biomimetic chemistry technique to construct high-performance porous carbon materials.Si C nanospheres are firstly coated with polydopamine(PDA)to form core-shell structured Si C@PDA precursor.Si C core and PDA shell are subsequently transformed into Si C-CDC core with rich nano pores and active N-doped carbon shell to form hierarchical porous CDC@NC through one-step molten salt electro-etching of Si C@PDA.Systematical investigation shows that the obtained CDC@NC has a high specific surface area of 1191 m²/g and typical core-shell structure.As expected,the highly active hierarchical porous core-shell structure is beneficial to the electrolyte permeability and ion transport/diffusion routes,and the CDC@NC exhibits a high specific capacity of 255 F/g and excellent cycling stability at the current density of 0.5 A/g.Even at the current density of 20 A/g,the CDC@NC can still deliver a high discharge capacity of 193 F/g,which is significantly better than that of the carbon materials prepared by conventional processes.In general,this integrated molten salt electrolytic production strategy provides a new method for the synthesis of highly active porous core-shell carbon materials.(4)Cr₂Al C with intrinsic nano-layered structure has been utilized as precursor to further synthesized 2D porous carbon by molten salt electrochemical etching process.It is found that electrolytic temperature has a great influence on the morphology and microstructure of the Cr₂Al C-CDC.Cr₂Al C-CDC prepared at 600 ^oC is mainly graphene-like amorphous carbon with abundant micro/meso-pores(0.94 cm³/g)and a high specific surface area(1343 m²/g),while high temperature is conducive to the transformation of amorphous carbon into ordered graphite-structure carbon.The Cr₂Al C-CDC obtained at 900 ^oC shows stacked layered porous graphite particles.In addition,the specific surface area of Cr₂Al C-CDC decreases with the increase of temperature and the pore size increases with increasing temperature,indicating that electrolytic temperature can be used to regulate the morphology and pore structure of Cr₂Al C-CDC.In the investiagtion of etching process of Cr₂Al C,it is found that the formation process of Cr₂Al C-CDC mainly includes electrochemical oxidation and etching processes.The oxygen residual in the melts results in the partial oxidation of Cr₂Al C and induces the formation of Al₂O₃ nanoparticles,however,it cannot influence the electrochemical etching process to obtain Cr₂Al C-CDC.As expected,the 2D Cr₂Al C-CDC prepared at 600 ^oC delivers a specific capacity of 183 F/g with the capacitive retention of 98.2%after 5000 cycles,showing excellent electrochemical stability.(5)Based on the above results,the molten salt electrochemical etching mechanism and adjustment strategy of carbide-derived carbon have been discussed systematically.The metal/metal-like elements with higher electrochemical activity can be etched from the carbide phase in the anodic electrochemical dealloying process.The kinetics of molten salt electrochemical etching process mainly includes two stages,i.e.,the surface electro-etching stage controlled by the electrochemical reaction and the internal electro-etching stage dominated by mass transfer process.The morphological and structured adjustment strategies of CDC materials mainly include the design and synergistic control of precursors,liquid molten salt and electrolysis parameters.The morphology of CDC can be preliminary controlled by reasonably designing the morphology of carbide precursors.The intrinsic crystal structure of carbides has a decisive influence on the final structure of CDC.Uniform distribution of carbon sites in the bulk carbide phase is beneficial to the generation of amorphous carbon.Furthermore,the liquid molten salt is helpful to the formation of hierarchical porous carbon.The electrolytic parameters would further affect the microstructure of CDC,meanwhile the amorphous carbon and graphitized carbon can be prepared by adjusting etching temperature in a controlled manner.